It is known, that when coupling a modem to a two-wire transmission line a fraction of the signal transmitted by the transmit path may occur inopportunely in the receive path of this modem owing to imperfections in the hybrid coupler coupling the modem to the transmission line or owing to reflections in this transmission line. An echo canceler has for its object to cancel this unwanted signal or echo signal appearing in the receive path. Canceling the echo has the advantage of permitting a full-duplex transmission between the modems interconnected by means of a two-wire transmission line.
An echo canceler of the above-described type is disclosed in U.S. Pat. No. 4,162,378. In accordance with this prior art technique, an echo canceler comprises in principle the same number of adaptive digital filters as there are distinct phase states of the transmitted carrier, each adaptive filter being assigned to a predetermined phase state. In each modulation interval, a number corresponding with the amplitude of the transmitted carrier is applied to an adaptive filter corresponding with the phase of the transmitted carrier, while the number 0 is applied to all other filters. In the majority of the customary modulation methods, one half of the phases of the transmitted carrier differs an amount of 180.degree. from the other half of the phases and the number of adaptive filters can be divided by two, each filter then begin assigned to a phase state .psi. and .psi.+180.degree. of the transmitted carrier, while the numbers corresponding with the amplitude of the carrier and assigned to each filter are provided in similar manner with a "+" sign or a "-" sign.
When this known technique is used, the adaptive digital filters can only be realized in a simple way in the case of phase modulation without amplitude modulation. Then the numbers applied to the input of the filter can be reduced to the values +1, -1 or 0 and the circuits used in the filters for multiplication by these numbers can be reduced to very simple logic circuits. In contrast therewith, when the carrier is amplitude-modulated as well as phase-modulated the adaptive digital filters must be realized in a more complex manner, particularly if the amplitude levels of the carrier have mutually "complicated" ratios such as 1/3 or .sqroot.2, which is often the case. The digital filters must then be provided with calculation circuits for performing multiplications by these "complicated" numbers.